Effect of convection heat transfer on the design of vapor-cooled current leads

A general optimization method for vapor-cooled current leads is presented with taking into account the effect of convection heat transfer and extended surfaces. This analytical work is considered as a unified design method, since one formulation calculates the minimum heat load and the corresponding optimal design condition for arbitrary heat transfer condition, spanning two limiting cases—the zero convection (or conduction-cooled leads) and the perfect heat transfer. It is clearly shown that the augmentation of the convective cooling can reduce the heat load to a certain extent, but the optimum lead parameter required to minimize the heat load for the finite heat transfer may not exist between the two limiting values. A new dimensionless parameter called the Ch number is introduced to conveniently incorporate the convection effect into the optimization. The present method is demonstrated for two specific lead designs that have been recently developed for 10 kA level of applications.     

A method of combined simulation of unsteady heat and mass transfer processes

A combined method is described for the electrical analog method of simulating heat and mass transfer processes, using two networks: On one network of capacitors and resistors, heat transfer is simulated; on the other, mass transfer, the cells of the networks being interconnected.     

Thermal Nondestructive Testing of the Thermal Properties of Materials Using Multifactor Transformation Functions

A solution of the direct problem of heat conduction when a linear pulsed heat source acts in the plane of contact of two semibounded bodies, obtained by the method of finite differences, is presented. The results of modeling are given. Using a discrete model, a method of determining (identifying) the thermal properties of materials using multifactor transformation functions is considered. __________ Translated from Izmeritel'naya Tekhnika, No. 7, pp. 44–49, July, 2005.     

两种CPU散热器换热特性的数值研究

对整体平直翅片与分段开缝平直翅片两种不同结构的CPU散热器，在不同加热功率和不同风速下的散热特性进行了数值模拟。采用商业软件FLUENT6．0进行计算，结果显示：对流换热表面传热系数和热阻主要与风速有关，与加热功率关系不大，对流换热表面传热系数的计算结果和实验数据趋势一致。在低风速下，加热功率对这两种散热器压降的影响也不大。采取数值模拟的方法对CPU散热器进行研究是实验方法的可靠补充，计算所得的结果有助于分析CPU散热器的换热过程，并可为电子设备的设计和改进提供参考。     

Effect of multi-stream heat exchanger on performance of natural gas liquefaction with mixed refrigerant

A thermodynamic study is carried out to investigate the effect of multi-stream heat exchanger on the performance of natural gas (NG) liquefaction with mixed refrigerant (MR). A cold stream (low-pressure MR) is in thermal contact with opposite flow of two hot streams (high-pressure MR and NG feed) at the same time. In typical process simulation with commercial software (such as Aspen HYSYS®), the liquefaction performance is estimated with a method of minimum temperature approach, simply assuming that two hot streams have the same temperature. In this study, local energy balance equations are rigorously solved with temperature-dependent properties of MR and NG feed, and are linked to the thermodynamic cycle analysis. The figure of merit (FOM) is quantitatively examined in terms of UA (the product of overall heat transfer coefficient and heat exchange area) between respective streams. In a single-stage MR process, it is concluded that the temperature profile from HYSYS is difficult to realize in practice, and the FOM value from HYSYS is an over-estimate, but can be closely achieved with a proper heat-exchanger design. It is also demonstrated that there exists a unique optimal ratio in three UA’s, and no direct heat exchanger between hot streams is recommended.     

The principles of information processing based on the numerical solution of the nonlinear heat-conduction problem. Pt. 1. The three-dimensional discrete mathematical model

A three-dimensional solution of the direct problem of heat conduction when a linear pulsed heat source acts in the plane of contact of two semibounded bodies is presented. The solution is obtained by the method of finite differences. An example of the modeling is considered. __________ Translated from Izmeritel’naya Tekhnika, No. 10, pp. 42–45, October, 2007.     

Heat flow visualization for thermal bridge problems

A method for the plotting of heat-flow intensity to provide an effective visual representation of heat flows in solid matter is proposed in this report. The heat-flow intensities are defined as the heat gain and the heat loss per volume. Two-dimensional steady-state simulation of heat flow in a structure was carried out under two conditions and the results were represented as plots of the temperatures and the heat-flow intensities. This representation of heat flow provides a stronger emphasis on the heat flows through good conductors than does the temperature distribution; the proposed method thus helps us to effectively visualize the effects of thermal bridges.     

Analytical Solution of Two‐Dimensional Contact Problems of Unsteady Heat Conduction in the Presence of Mixed Boundary Conditions in the Contact Plane

A method for solution of systems of parabolic differential equations of heat conduction on the model of thermal contact between two bodies with different thermophysical characteristics in the presence of mixed boundary conditions in the plane of their contact has been suggested for the first time. The case of contact of two semibounded bodies has been considered. In this case, a heat source of low heat capacity acts in a circular region of finite radius on the contact surface, and beyond this region the initial temperature is maintained during the whole period of heat transfer.     

竖直U型地热换热器流体出口温度的计算模型

以圆柱热源理论为基础,引入“近热史”和“远热史”的概念,采用简化的方法建立了变热流情况下换热器与周围土壤的传热计算模型,利用该模型在已知远地点温度Tg的情况下可求解出钻孔壁温度Tw。随后引入形状因素的概念,建立了管内稳定传热的计算模型,结合前述求得的Tw,得出了计算换热器内循环流体的出口温度Tout的方法。     

Heat transfer and pressure drop in a plate-type evaporator

A plate-type evaporator, working with natural refrigerant circulation, has been investigated both experimentally and theoretically. Motivated by the phase-out of ozone-depleting substances, HCFC22 was compared to HFC134a and two zeotropic refrigerant mixtures. The effect of different separator liquid levels, i.e. refrigerant flows, and its influence on heat transfer was also studied. The investigated plate-type evaporator consists of thirteen vertical flow channels and its size is 3.0 m × 0.5 m. The heat source for the evaporator is a falling water film on the outside of the plate. Experimental studies have been carried out using a test facility that enabled detailed measurements of heat transfer and pressure drop. Experiments were compared to results from a calculation method that simultaneously calculates heat transfer and pressure drop in a variable number of steps along the evaporator. The calculation method is based on a pressure drop correlation proposed by the VDI-Wärmeatlas and a heat transfer correlation for vertical tubes proposed by Steiner and Taborek. For different evaporator duties, heat transfer was over predicted by 12% for pure fluids by 15% for mixtures. Calculated pressure drops were well within ±5% of the measured values. Changes in heat transfer due to different flows were closely predicted by the proposed calculation method.     

On the heat-flow distribution from and the temperature profile on an equally heated rectangle,calculated by series development

A method is proposed to approximate the solutions of a certain class of differential equations, linear or nonlinear, in two or three, dimensions, provided that the boundary conditions are given on a rectangle or a parallelepiped, respectively. For other boundary shapes the co-ordinate system must be transformed to meet that requirement. As in illustration, an example is given for the solution of Poisson's equation in two dimensions with a constant heat source, giving the temperatures on the rectangle together with the heat-flow distribution along its edges. The basis of the method is a Taylor-series development around one point; the result is given in terms of as many partial derivatives in that point as is desired. A similar method has already been described by Small² for the ‘heat equation’ ?θ/?t = ?²θ/?x²². Compared with finite difference and finite element solutions, these methods have the advantage that the solution is continuous, whereas first and second derivatives such as heat fluxes are available at hardly any effort. The results are compared with those of the exact solution. Even if the size of the determinant is limited to 4 × 4, the accuracy is already better than 98.98 per cent. When more effort is spent to solve a system of 10 equations, the accuracy is better than 99–95 per cent.     

Two‐dimensional unsteady heat conduction analysis with heat generation by triple‐reciprocity BEM

If the initial temperature is assumed to be constant, a domain integral is not needed to solve unsteady heat conduction problems without heat generation using the boundary element method (BEM).However, with heat generation or a non‐uniform initial temperature distribution, the domain integral is necessary. This paper demonstrates that two‐dimensional problems of unsteady heat conduction with heat generation and a non‐uniform initial temperature distribution can be solved approximately without the domain integral by the triple‐reciprocity boundary element method. In this method, heat generation and the initial temperature distribution are interpolated using the boundary integral equation. Copyright © 2001 John Wiley & Sons, Ltd.     

Monte Carlo simulations of magnetic properties in multilayers

A Monte Carlo method has been used to simulate Heisenberg multilayer systems (L × L × 4P) consisting of alternating P ferromagnetic layers A and B with antiferromagnetic interface coupling J_AB. Finite-size effects on the specific heat and magnetisation thermal variation for two kinds of boundary conditions at the top and bottom planes are investigated. In particular, our Monte Carlo data evidence that the specific heat exhibits two peaks and a single phase transition occurs at the temperature which corresponds to the location of the high temperature peak (as L → ∞).     

Measurement and prediction of evaporator shell-side pressure drop

The pressure drop across a heat exchanger is an important parameter, along with the heat transfer capacity. In fact, the operating cost throughout the life of the exchanger depends on the pressure losses. Therefore, it is important to be able to predict pressure drop accurately as it is to predict heat transfer.A new data set of shell-side pressure drop measurements taken during isothermal flow of brines in shell and tube evaporators was collected in the Alfa Laval laboratory. It covers several different configurations of industrial shell and tube evaporators and a wide range of operating conditions, with cross flow Reynolds number ranging from 170 to 33,000.The database is compared against two predictive procedures available in the literature for computing shell-side pressure drop, showing that no method is accurate enough for design purpose.As a further step, a new suggested procedure is presented, which extends the Wills and Johnston [Wills MJN, Johnston D. A new and accurate hand calculation method for shellside pressure drop and flow distribution. 22nd National Heat Transfer Conference, HTD N. 36. New York: ASME; 1984, p. 67–79] method to the low Reynolds number range and improve its capability to predict experimental data.     

Carnot comparison of multi-temperature level absorption heat cycles

Comparison of different absorption heat cycles is not always made on the correct manner. This also includes comparison of an ideal absorption cycle with a mechanical analogy. A new Carnot model operating with two heat engines and two mechanical heat pumps is defined to be the correct and logical way to describe the mechanical analogy for an absorption heat pump and an absorption heat transformer. General equations for the Carnot coefficient of performance, COP_r, are exemplified and simulated for an absorption heat pump and an absorption heat transformer, and an entropy flow fraction diagram is introduced. The important fact that the absorption heat cycles must operate under the same conditions when they are compared is discussed.     

二维多孔材料散热性能分析与设计

二维多孔材料存在一个易于流动的方向并具有较大的面密度 , 因此在具有良好的比刚度和比强度的同时也具有良好的散热性能 , 研究强迫对流下的散热性能对其多功能化设计具有重要意义。本文中利用数值方法求解考虑二维多孔材料内部流体流动规律、 热传导和对流换热影响的流固耦合热传输问题 , 分析了多孔率和微结构尺寸对散热性能的影响并进行了最优参数设计 ; 通过分析比较 5种具有典型微结构形式的二维多孔材料的散热性能 , 给出了微结构形式对散热性能的影响。提出了以需要满足的散热性能为约束条件 , 以满足需求的设计参数的可调范围(设计参数的允许变化范围)为设计目标的最优散热结构设计理念。以此理念得到的设计结果 , 更有利于根据其他性能的要求对材料进行多功能化设计。分析表明 , 具有正六边形微结构的二维多孔材料的散热性能最优 , 并有利于实现轻质多功能化设计。     

Thermal diffusivity measurement of CVD diamond film using a step heating technique

A step heating method for the measurement of the thermal diffusivity of diamond thin film is described. The step heating method is a transient heat flow method. Transient temperature profiles are generated in a strip-shaped sample by heating one end of the sample while the other end is clamped to a heat sink. Three thermocouples are used along the heat path. The results are compared with the literature values over the temperature range from –190 to 50°C.     

一种非连续介质中热传导过程的数值模拟方法

传统热传导的分析基于连续模型,无法刻画热量在两个接触体之间的传递。该文提出了一种非连续介质中热传导过程的数值计算方法,并编制了相应的C++计算程序。该方法首先将计算域离散为一系列的块体,块体内部划分若干连续介质单元,块体边界设定为潜在接触界面,并利用半弹簧-半棱联合接触模型进行接触对的快速检索及标记。每个块体内部的热传导采用传统连续模型进行计算(该文采用有限体积法),每个接触界面采用点面接触型及棱棱接触型热传导模型进行描述。通过调整接触界面热传导系数中的刚度因子,可以实现接触界面对热传导过程不同的抵抗效应。数值算例表明,该文所述方法可以较为准确地模拟热量在非连续介质中的传递过程;接触界面上的刚度因子越大,界面对热传导过程的抵抗效应越小;当刚度因子大于100,界面抵抗效应基本消失,非连续介质的计算结果与连续介质的计算结果完全一致;此外,接触界面上的刚度因子仅影响热传导的瞬态过程,而不影响其稳态解。     

Numerical analysis of evaporation performance in a finned-tube heat exchanger

This study discusses the effects of the heat exchanger type, refrigerant, inner tube configuration, and fin geometry on evaporator performance by adopting updated correlations of EVSIM, a numerical analysis model based on the tube-by-tube method developed by Domanski. The heat exchanger types considered are the cross-counter flow type and cross-parallel flow type. The refrigerants considered for the numerical test as a working fluid are R-134a, R-410A and R-22. For inner tube configuration, enhanced tube and smooth tube cases are considered. For the air side evaporation performance, heat exchangers using plate fins, wavy fins and slit fins are analyzed. Results show that the heat transfer rate of the cross-counter flow type heat exchanger is 3% higher than that of the cross-parallel flow type with R-22. The total heat transfer rate of the evaporator using R-410A is higher than those using R-22 and R-134a, while the total pressure drop of R-410A is lower than those of R-22 and R-134a. The heat transfer rate of the evaporator using enhanced tubes is two times higher than that using smooth tubes, but the pressure drop of the enhanced tube is 45–50% higher than that of the smooth tubes. The evaporation performance of slit fins is superior to that of plate fins by 54%.